Integrated optical waveform modulation

ABSTRACT

A method of modulating an optical carrier. A target carrier modulation is computed based on an input data signal. An effective length of an optical modulator is then controlled based on the target carrier modulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/357,963 filed Feb. 22, 2006.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to optical signal transmitters for opticalcommunications systems, and in particular to integrated optical waveformmodulation.

BACKGROUND OF THE INVENTION

In the optical communications space, various techniques are used tosynthesize an optical communications signal for transmission. A populartechnique utilizes a laser 2 coupled to an external optical modulator 4,as shown in FIG. 1. The laser 2 generates a narrow-band continuous wave(CW) optical carrier signal 6 having a desired wavelength. The opticalmodulator 4 operates to modulate the amplitude and/or phase the carriersignal 6 to generate the optical communications signal 8 based on adrive signal 10 that encodes data to be transmitted. Typically, thedrive signal 10 is generated by a driver circuit 12, which normallyprovides a power amplifier for amplifying the power of an input digitaldata signal x(m) to satisfy the input power requirements of themodulator 4.

In the arrangement illustrated in the FIG. 1, the optical modulator 4 isprovided by a well known Mach-Zehnder (MZ) interferometer. Other typesof modulators may be used, depending on the desired type of modulation.For example, an electro-absorptive modulator (EAM) may be used foramplitude modulation; whereas phase modulators are well known forimplementing phase modulation. In each case, the driver circuit 12generates the drive signal 10 by scaling the input data signal x(t) tosatisfy the voltage and current requirements of the modulator 4. Thedriver circuit 12 may also generate one or more bias signals (not shown)for controlling a bias point of the modulator 4 in a manner well knownin the art. The amount of modulation achieved is proportional to theproduct of effective length of the modulator and the drive voltageapplied. The length of the modulation element is fixed by theconstruction of the modulator, and the drive voltage is adapted toachieve the desired amount of modulation. Control circuits are generallyused to achieve this adaptation.

FIG. 2 illustrates an alternative arrangement known from Applicant'sco-pending U.S. patent application Ser. No. 10/677,223 filed Oct. 3,2003. In that system, a complex driver circuit 14 comprises a digitalfilter 16 which uses the input data signal x(m) and a compensationfunction c(t) to calculate multi-bit In-Phase and Quadrature componentvalues I(n) and Q(n) of a target optical E-field modulation. Anon-linear compensator 18 uses the I(n) and Q(n) components to computemulti-bit sample streams V_(R)(n) and V_(L)(n). These digital samplestreams are then converted into corresponding analog voltage levels byrespective multi-bit digital-to-analog converters (DACs) 20, filtered(at 22) to reduce out-of-band noise, and scaled by low noise amplifiers24 to yield a pair of drive signals V_(R)(t) and V_(L)(t) which aresupplied to respective branches of the MZ modulator 4. If desired,respective digital filters (not shown) may be positioned between thenon-linear compensator 18 and the DACs 20 in order to compensate anypropagation delay differences between the DACs 20 and the MZ modulator4.

As may be seen in FIG. 3 a, an electro-optic component typicallyincludes a control region 26 defined by a pair of electrodes 28 placedon opposite sides of an optical waveguide 30. With this arrangement, anelectric field through the waveguide material can be set up by applyinga voltage across the two electrodes 28. For High-speed applications(i.e. for micro-wave frequency drive signals), the electrodes 28 aretypically implemented using strip-line techniques, with the drive signalv(t) supplied to the up-stream end of the electrode 28, relative to thedirection of propagation of light through the waveguide 30. The opposite(downstream) end of the electrode 28 is typically terminated by amatched resistive load to ground (not shown) to prevent unwanted signalreflections. Ideally, the characteristic impedance of the electrode 28is selected such that the drive signal propagates through the electrode28 at the same speed as the light propagating in the optical waveguide30.

The arrangement shown in FIG. 3 a is typical of a voltage-controlledphase modulator. In such a phase modulator, the refractive index of thewaveguide is a function of the applied voltage, so that lightpropagating though the control region 26 will experience a phase delaythat is proportional to the applied voltage and the length L of theelectrodes 28. In order to maximize the phase delay, the drive signalV(t) is commonly supplied as a differential signal pair ±V(t), whichdoubles the magnitude of the voltage across the electrodes 28.

FIG. 3 b shows a dual-branch MZ modulator 4 constructed using a pair ofphase modulators of the type illustrated in FIG. 3 a. For simplicity,bias control circuits which are normally provided as part of the MZmodulator 4 are not shown. Voltage inverters 32 in each drive signalpath convert the applied drive signals V_(x)(t) into correspondingdifferential voltage pairs ±V_(x)(t). If desired, the drive signalsV_(x)(t) can be generated by the complex driver 14 described above withreference to FIG. 2.

The arrangement of FIGS. 2-3 b is particularly advantageous in that themulti-bit sample values V_(R)(n) and V_(L)(n) can be computed takinginto account non-linearities of the analog signal path (e.g. the DACs20, filters 22 and LNAs 34) and the MZ modulator 4, such that theoptical E-field of the composite signal 8 appearing at the output of theMZ modulator 4 closely matches the target E-field modulation computed bythe digital filter 16. In some embodiments, the compensation functionc(t) is selected to compensate impairments of an optical link not shown,in which case the target E-field modulation represents a pre-distortedsignal which will be transformed by the link impairments into asubstantially undistorted optical signal at a receiver end of the link.

As will be appreciated, at least the digital filter 16, non-linearcompensator 18 and DACs 20 of the complex driver 14 can be implementedon a single Application Specific Integrated Circuit (ASIC). Thisarrangement provides advantages in terms of performance, powerconsumption and cost. For example, in some embodiments, an ASICimplemented using Complementary Metal Oxide Semiconductor (CMOS)technology can cost-effectively generate the analog DAC output signalsto 6-bits precision at a sample rate of 20 GHz. This performance issufficient to compensate even severe impairments of the optical link atdata rates of 10 Gb/s.

However, electro-optical components such as MZ modulators are typicallyfabricated using techniques that are not readily compatible with thoseof integrated circuits (ICs). In many cases, the materials used forelectro-optical components differ from those used in IC fabrication, orfrom the preferred technology (e.g. CMOS, GaAs etc.) of the driver IC,which would require redesign of the IC production line. Even where thisis done, the drive signals V(t) required to obtain satisfactory dynamicrange of the electro-optical component may significantly exceed thevoltage and/or thermal limits of the IC. For example, in the system ofFIG. 2, each of the drive signals V_(R)(t) and V_(L)(t) may require apower level on the order of 10 Watts or more.

In view of the above difficulties, the driver 12 or 14 and the opticalmodulator 4 are typically provided as separate packages mounted on aprinted circuit board, separated by the signal conditioning components(e.g. filters and amplifiers) required to modify the drive signalsoutput from the driver ASIC to satisfy the input power requirements ofthe optical modulator 4. In many cases, this is a satisfactoryarrangement. However, in some applications it is desired to provide asmaller, lower power assembly.

U.S. Pat. No. 4,288,785 (Papuchon et al) teaches a digitally controlledlight intensity (amplitude) modulator which is compatible withTransistor-Transistor Logic (TTL). According to Papuchon et al, each bitof an N-bit control word is used to control the voltage supplied to arespective electrode of an MZ modulator. In one embodiment, theelectrodes are arranged symmetrically on both branches of the MZmodulator, and receive the same control signal. In other cases, theelectrodes are arranged on one branch of the MZ modulator, while theother branch receives a feed-back signal designed to linearize the MZmodulator response. In some embodiments, the length of each electrodevaries in a geometric scale according to the binary weight of itscontrol bit. In other embodiments, each electrode has the same length,and voltage dividers are used to scale the voltage supplied to eachelectrode, again according to the binary weight of its control bit.

It will be appreciated that the modulator of Papuchon et al is a highvoltage device designed to operate at relatively low speeds. Inparticular, the modulator is driven by TTL logic, which produces a 5volt swing between binary ‘0’ and ‘1’ logic states. It is also wellknown that TTL logic is a relatively high-current and low yieldtechnology, which precludes its use in large integrated circuits withclock speeds higher than about 10⁶ Hz. However, high speed integratedcircuits, for example high speed CMOS IC's operating at clock speedsexceeding 10⁹ Hz, typically have a voltage swing on the order to 1 volt,and are capable of sourcing only very limited currents. This leads backto an assembly in which the driver IC and the optical modulator 4 mustbe provided as separate packages mounted on a printed circuit board,separated by the signal conditioning components (in this case highvoltage drivers). In addition, the feedback loop used to linearize thesinusoidal response of the MZ modulator includes an inherent loop delay.This loop delay imposes an upper limit on the speed of the control word,beyond which the MZ modulator response will become increasinglynon-linear.

Because Papuchon is considering low speed modulations with an idealizedMZ modulator, he does not indicate methods to mitigate or avoid thenonidealities of an actual modulator operating at high speeds, withphysical limitations and imperfect manufacturing.

Digitally driven optical modulators capable of high-speed operation areknown, for example, from A 12.5 GSample/s Optical Digital-to-AnalogConverter with 3.8 Effective Bits, A. Leven et al., Lasers andElectro-Optics Society, 2004, and references cited therein. In eachcase, multiple parallel phase modulators are provided, each of which iscontrolled by a respective drive voltage v_(i), which appears to bescaled to follow the binary weight of a respective bit of a digitalcontrol signal so as to impose a phase shift proportional to the binaryweight of that bit. The signals emerging from the parallel phasemodulators are coherently combined to yield a composite amplitudemodulated optical signal, the intensity of which is proportional to thevalue of the digital control signal.

An alternative arrangement is described in Digital-to-analog ConversionUsing Electrooptic Modulators, Yacoubian et al, IEEE PhotonicsTechnology Letters, Vol 15, No. 1, January 2003. In this case, multipleparallel amplitude modulators are provided. Each modulator is driven bya respective bit of a digital control signal, and attenuates light inresponse to the logic state of that bit. Binary-scale weighting of eachamplitude modulator is obtained by using a weighted 1-to-N coupler whichdivides a continuous wave (CW) carrier to produce the branch signalssupplied to each controlled amplitude modulator. The signals emergingfrom the parallel amplitude modulators are in-coherently combined on aphotodetector to yield an electrical signal having an amplitude that isproportional to the value of the digital control signal.

Unlike the system of Papuchon et al, the systems of Leven et al. andYacoubian et al are capable of operating at high speeds. The system ofYacoubian et al is also capable of operating with sub-1 volt controlsignals, and thus cold be driven by a high speed CMOS IC, for example.However, neither of these systems is suitable for an optical synthesizerin which the driver IC and optical modulator are integrated within acommon package. In particular, the system of Leven requires a drivecircuit capable of delivering a scaled voltage to each phase modulator,which precludes direct connections between the phase modulators and adriver IC. In the case of Yacoubian, the signals emerging from theoptical modulators must be recombined incoherently in order to avoiderrors. This precludes operation as an optical synthesizer, because itcannot reliably generate a low-error optical signal.

Accordingly, methods and apparatus enabling a high speed driver IC andan optical modulator to be integrated within a common package remainhighly desirable.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide methodsand apparatus enabling a driver IC and an optical modulator to beintegrated on a common substrate.

Thus, an aspect of the present invention provides a A method ofmodulating an optical carrier. A target carrier modulation is calculatedbased on an input data signal. An effective length of a control regionof an optical modulator is then controlled based on the target carriermodulation.

Another aspect of the present invention provides an optical synthesizer.The synthesizer includes a driver integrated circuit (IC) for computinga target carrier modulation based on an input data signal, an opticalmodulator co-packaged with the driver IC, and means for controlling aneffective length of a control region of the optical modulator based onthe target carrier modulation.

The optical synthesizer may, for example, be incorporated into a systemfor transmitting a data signal through an optical network. Such a systemmay, for example, take the form of a modem, line card, or network nodeincorporating such devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 schematically illustrates principal components and operation of aone-dimensional communications signal synthesizer known in the priorart;

FIG. 2 schematically illustrates principal components of a complexoptical synthesizer known from applicant's co-pending U.S. patentapplication Ser. No. 10/677,223 filed Oct. 3, 2003;

FIGS. 3 a and 3 b schematically illustrate respective electro-opticalcomponents;

FIG. 4 schematically illustrates principal components and operation of acomplex optical synthesizer in accordance with an embodiment of thepresent invention;

FIGS. 5 a and 5 b schematically illustrate a representative MZ modulatorhaving a first electrode arrangement usable in the embodiment of FIG. 4;

FIG. 6 schematically illustrates an alternative electrode arrangementusable in the embodiment of FIG. 4;

FIG. 7 schematically illustrates principal components and operation of acomplex optical synthesizer in accordance with a second embodiment ofthe present invention; and

FIG. 8 schematically illustrates a representative MZ modulator usable inthe embodiment of FIG. 7.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides methods and apparatus for modulating theE-field of an optical carrier signal, in which the driver IC and theoptical modulator can be integrated within a common package (i.e.co-packaged), either on a common substrate or on separate substratesclosely coupled together. Embodiments of the invention are describedbelow, by way of example only, with reference to FIGS. 4-7.

In general, the present invention operates by computing a target carriermodulation, and then varying the effective length of the control region26 of the optical modulator in accordance with the target modulation.The electrodes of the modulator are configured such that the drivesignals are the binary logic states output by the driver IC with nosignal conditioning or power amplification required between the driverIC and the optical modulator. FIG. 4 illustrates a first representativeembodiment of the present invention.

In the embodiment of FIG. 4, the driver IC is implemented as a digitalsignal processor (DSP) 34, which generates a pair of multi-bit samplestreams V_(X)(n) which are representative of the desired phasemodulation to be applied to each branch of an MZ modulator 4.Advantageously, the driver IC and modulator 4 are designed to facilitateco-packaging. In some embodiments, this is accomplished by implementingboth the driver IC and modulator 4 on a common substrate using the sameIC technology. For example, Indium-Phosphide (InP) or Galium-Arsenide(GaAs) processes can be used to construct both the driver IC and electrooptical components on a common substrate. Various known methods can beused to electrically connect the driver IC to the electro opticalcomponent, such as IC circuit traces or wire bonding.

Alternatively, the driver IC and electro optical components can beconstructed on separate chips or wafers, and using respective differentprocesses that are compatible for co-packaging. In such cases, the twocomponents can be physically secured in either a tiled or stackedarrangement within a common package, and electrically connected usingvarious known methods, such as wire bonding of solder balls.

With either of the above arrangements, close connection of the driver ICand electro optical components within a common package allows multipleparallel lines to be cost effectively constructed with short lengths andthe precision and control needed to provide impedance matching anddesired differential propagation delays.

It should be noted that the illustrated embodiments utilize an MZmodulator. However, other types of electro-optical devices may equallybe controlled using the methods of the present invention. For Example,other types of interference modulators can be used. Absorptionmodulators, such as Electro Absorptive Modulators (EAMs) can be used,and benefit from digital precompensation of the non-ideal phase andabsorption characteristics by the DSP 34. Similarly, Nonlinear opticaland electrical elements can be incorporated into a more complicatedmodulation function. Four branch, reflective, or parallel or seriescombinations can also be used. The source of some or all of themodulation or control can be electrical or optical.

If desired, the DSP 34 may incorporate the functionality of the digitalfilter 16 and non-linear compensator 18 of the complex driver 14described above with reference to FIG. 2. This arrangement isadvantageous in that the digital filter 16 can be used to compute thedesired target modulation, and the non-linear compensator 18 used tocompensate non-linearities of the optical modulator 4. This cancompensate non-ideal electrical to optical transfer functions from anelectrode or electrodes, and thus at least partially compensatemanufacturing variations. Each multi-bit sample stream V_(X)(n) may bean N-bit parallel binary signal output from the DSP 34 on acorresponding N-bit data bus 36. In such a case, each line 38 _(i) ofthe N-bit bus 36 is connected to control a number of electrodes 40corresponding to its binary weight. For example, the least significantbit (LSB) of the of the multi-bit sample stream has a binary weight of“1”, and thus controls a single electrode 40 of the MZ modulator 4. Thei^(th) bit has a binary weight of 2^(i−1), and thus controls 2^(i−1)electrodes 40. This arrangement would apply to embodiments in which allof the electrodes are of substantially identical length, within normalmanufacturing tolerances. It is possible for at least some of theelectrodes to have different lengths, provided that the current drain ofeach electrode remains within the limits of the driver IC. FIGS. 5 and 6illustrate electrode connection schemes which implement this arrangementin more detail.

FIG. 5 a illustrates a branch of an MZ modulator 4 (or, equivalently avariable phase modulator) having an electrode arrangement that may beused in conjunction with the complex optical synthesizer of FIG. 4. Inthe embodiment of FIG. 5 a, each line 38 _(i) of the N-bit bus 36controls 2^(i−1) electrodes 40, each of which is composed of astrip-line element coupled to the data bus and terminated by a matchedresistive load (not shown). Because the electrodes 40 are being drivenby the driver IC output, the electrodes 40 should be designed to preventexcessive current drain. As may be seen in FIG. 5 a, buffers 42 can beused both to avoid excessive current drain on the driver IC and toimpose desired propagation delays between adjacent electrodes. FIG. 5 billustrates an alternative arrangement, in which the electrodes 40 areprovided as un-terminated capacitive pads. In the illustratedembodiment, the capacitive pads are constructed to overlie the waveguide30 so that an electric field can be set up through the waveguide 30between each capacitive pad and counter electrode 43 formed by a groundplane of the wafer.

As mentioned above in reference to FIG. 3, in a conventionalelectro-optical component the propagation speed of the drive signal V(t)through an electrode 28 is matched as closely as possible to that of theoptical signal through the waveguide 30. In the embodiment of FIG. 4,this effect can be emulated by inserting delays such that the time ofarrival of each bit of the drive signal at its set of electrodes 40coincides with the expected arrival time of an optical wave-frontpropagating through the waveguide 30. On a course level, this can bedone by suitably setting the length of each line of the parallel databus 36. Within each set of 2^(i−1) electrodes, propagation delaysbetween adjacent electrodes can be used to accomplish the requiredsequential time of arrival. As mentioned previously, buffers 42 can beused for this purpose, either alone or in combination with differentialsignal path lengths.

The timing of drive signal bits at each set of electrodes 40 isadvantageously fixed by the bus design, which determines the respectivepropagation delay of each bit between the driver IC and the MZ modulator4. However, these propagation delays can be variable. For example, thedelays may be set as part of a factory calibration of the driverIC/modulator package. Alternatively, a training or feed-back loop couldbe used to adjust line delays, either occasionally or at regularintervals.

As may be seen from FIG. 4, the number of “active” electrodes, and thusthe effective electrode length within each branch of the MZ modulator 4,will vary directly with the binary value of the corresponding multi-bitsample stream V_(X)(n). Since each active electrode receives the samevoltage (corresponding to logic state ‘1’), it follows that the totalphase delay experienced by light traversing each branch will varydirectly with the number of active electrodes on that branch, and thusthe value of the corresponding multi-bit sample stream V_(X)(n).

In the embodiment of FIG. 5, the counter electrode 43 is provided by acommon ground plane which extends along the entire length of eachbranch. This arrangement has an advantage of simplicity, and is suitablein circuits in which a differential voltage pair at each electrode 40cannot be readily generated, for example because a negative supplyvoltage (i.e. −Vdd) is not available.

FIG. 6 illustrates an alternative electrode arrangement, which may beimplemented in CMOS, for example, in which a pair of opposed electrodesare connected to +Vdd and −Vdd supply rails via respective transmissiongates 44 (or bilateral switches). As is known in the art, eachtransmission gate is composed of complementary N-type and P-type MOSFETscoupled “back-to-back” in such a way that an analog signal can beswitched by a common binary gate signal. Accordingly, the arrangement ofFIG. 6 enables a binary control signal (having voltages of GND and +Vdd)output from the driver IC 34 to supply a corresponding differentialvoltage ±Vdd to the opposed electrodes 40. Other methods of generating adifferential voltage pair will be apparent to those of ordinary skill inthe art.

FIGS. 7 and 8 illustrate an alternative embodiment in which the driverIC is implemented as a digital signal processor (DSP) 34 cascaded with apair of logic circuits 46. Each logic circuit 46 is designed to output adigital drive signal S_(x)(n) to a multi-bit parallel bus 48, each lineof which is connected to a respective electrode 40 of a dual-branch MZmodulator 4. Preferably, each electrode 40 is substantially identical,subject only to manufacturing variations.

If desired, the DSP 34 may incorporate the functionality of the digitalfilter 16 and non-linear compensator 18 of the complex driver 14described above with reference to FIG. 2. In such cases, the DSP 34 maygenerate a pair of multi-bit sample streams V_(X)(n) which arerepresentative of the desired phase modulation to be applied to eachbranch of the MZ modulator 4. Each logic circuit 46 uses the binaryvalue of each successive sample V_(X)(n) to output a logic state ‘1’ onthe corresponding number of lines of the parallel bus 48. For example,in an embodiment in which the multi-bit sample streams V_(X)(n) are 3bits wide (or only the 3 most significant bits—MSBs—are used) each logiccircuit 46 may implement the truth table of table 1 below. TABLE 1V_(x)(n) 3MSBs S_(x)(n) 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 1 0 11 0 0 0 0 0 1 1 0 1 1 1 0 0 0 0 0 0 1 1 1 1 1 0 0 0 1 0 1 1 1 1 1 1 0 00 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 1 1

As may be seen from table 1, the number of “active” electrodes, and thusthe effective electrode length within each branch of the MZ modulator 4varies directly with the binary value of the corresponding multi-bitsample stream V_(X)(n). Since each active electrode receives the samevoltage (corresponding to logic state ‘1’), it follows that the totalphase delay experienced by light traversing each branch will varydirectly with the number of active electrodes on that branch, and thusthe value of the corresponding multi-bit sample stream V_(X)(n).

As will be appreciated, desired time-of arrival delays of each bit ofthe drive signal Sx(n) can be obtained by suitably setting the length ofeach line of the parallel bus 48.

FIG. 8 illustrates the MZ modulator of FIG. 7 in greater detail, for thecase of simple electrodes of the type described above with reference toFIG. 5. It will be appreciated that other electrode types, such as thosedepicted in FIG. 6, for example, can be connected in a directlyanalogous manner.

The foregoing embodiments utilize a binary sequence of effectivelengths, and with equal electrode lengths, but other patterns can beused. The nonlinear compensator 18 can be used to compensate for otherpatterns, whether deliberate, or from processing variations. Forexample, the nonlinear compensator 18 can be used to compensateimbalance of the MZ modulator resulting from a number of “weak”electrodes which may be produced, for example, by a manufacturing flaw.

As described above, the control of the effective length of theelectrodes is most advantageously accomplished with nominally equaldrive voltages. However, it is also possible to use distinct drivelevels, or continuous analog drive voltages, such as from a traditionalhigh frequency modulator driver. A mixture of these different methodscan be used at once.

In the embodiments of FIGS. 4-8, separate bus lines 381 are shown foreach electrode, or each set of 2^(i−1) electrodes, which providesmaximum flexibility. However, bus lines can be bundled or coupled, forexample, in order to ease implementation.

Control of the effective length of the control region 26 isadvantageously obtained with selection of a plurality of electrodes bybinary logic. However, logic bases other than binary can be used, ifdesired. The change in effective length of the control region 26producing an electro-optic effect can be obtained by other methods, suchas by controlling the path of the light or the shape of the electricfield produced by one or more electrodes, or other biasing, so as tochange the overlap between the electric field, the light, and theelectro-optic material.

The embodiment(s) of the invention described above is(are) intended tobe representative only. The scope of the invention is therefore intendedto be limited solely by the scope of the appended claims.

1. A method of modulating an optical carrier, the method comprisingsteps of: computing a target carrier modulation based on an input datasignal; and controlling an effective length of a control region of anoptical modulator based on the target carrier modulation.
 2. A method asclaimed in claim 1, wherein the step of computing a target carriermodulation comprises a step of computing a multi-bit digital samplestream indicative of the target modulation.
 3. A method as claimed inclaim 2, wherein the multi-bit digital sample stream comprises arespective multi-bit digital sample stream for each branch of a dualbranch Mach-Zehnder (MZ) modulator, and wherein each multi-bit digitalsample stream represents a corresponding branch phase modulation whichwill yield the target carrier modulation in a composite optical signalgenerated at an output of the MZ modulator.
 4. A method as claimed inclaim 2, wherein the step of computing the multi-bit digital samplestream comprises steps of: computing respective multi-bit digital valuesof In-phase and Quadrature components of a target optical E-fieldmodulation; and computing the multi-bit digital sample stream based onthe In-phase and Quadrature component values.
 5. A method as claimed inclaim 4, wherein the step of computing multi-bit digital values of atarget optical E-field modulation comprises a step applying acompensation function adapted to compensate impairments of an opticallink.
 6. A method as claimed in claim 2, wherein the step of computing amulti-bit digital sample stream comprises a step of a compensatingnonlinearity of the modulator.
 7. A method as claimed in claim 1,wherein the step of controlling an effective length of the controlregion of an optical modulator comprises a step of compensating anelectrode characteristic of the MZ modulator.
 8. A method as claimed inclaim 7, wherein the electrode characteristic any one or more of:electrode length; bandwidth; transfer function; and delay.
 9. A methodas claimed in claim 7, wherein the step of compensating the electrodecharacteristic is performed digitally.
 10. A method as claimed in claim7, wherein the step of compensating the electrode characteristic isperformed using an analog compensation function.
 11. A method as claimedin claim 7, wherein the step of compensating the electrodecharacteristic comprises applying a linear compensation function.
 12. Amethod as claimed in claim 7, wherein the step of compensating theelectrode characteristic comprises applying a non-linear compensationfunction.
 13. A method as claimed in claim 2, wherein the step ofcontrolling an effective length of the control region comprises stepsof: providing a plurality of electrodes in operative relation to awaveguide of the optical modulator, each electrode being adapted toimpose a respective predetermined modulation to light propagating withinthe waveguide; and selectively activating one of more of the pluralityof electrodes in accordance with the target carrier modulation.
 14. Amethod as claimed in claim 13, wherein each electrode is substantiallyidentical.
 15. A method as claimed in claim 13, wherein the plurality ofelectrodes comprises a respective set of one of more electrodes for eachbit of the multi-bit digital sample stream, the number of electrodes ofeach set corresponding to a binary weight of its respective bit of thedigital sample stream.
 16. A method as claimed in claim 15, wherein thestep of selectively activating one of more of the plurality ofelectrodes comprises a step of supplying each bit of the multi-bitdigital sample stream to its respective set of electrodes.
 17. A methodas claimed in claim 16, further comprising a step of differentiallydelaying each bit of the multi-bit digital sample stream in accordancewith a propagations speed of an optical signal through the waveguide.18. A method as claimed in claim 16, further comprising, for each set ofelectrodes, a step of delaying a time of arrival of the respective bitof the sample stream to each electrode of the set, such that a time ofarrival of the bit at each electrode substantially corresponds with anexpected time of arrival of an optical wavefront propagating through thewaveguide.
 19. A method as claimed in claim 13, wherein the step ofselectively activating one of more of the plurality of electrodescomprises steps of: computing a number of electrodes corresponding to abinary value of the multi-bit digital sample stream; and supplying acontrol signal to the computed number of electrodes.
 20. An integratedoptical synthesizer comprising: a driver integrated circuit (IC) forcomputing a target carrier modulation based on an input data signal; anoptical modulator co-packaged with the driver IC; and means forcontrolling an effective length of a control region of the opticalmodulator based on the target carrier modulation.